1. Field of the Invention
This invention relates to line type modulators, and more particularly, variable pulse width, constant energy line type modulators utilizing a pulse forming network.
2. Description of the Prior Art
In the prior art, the pulse width of the line type modulator could be altered by discharging part of the energy in the pulse forming network through a dummy load. One end of the pulse forming network would be attached to the load while the other end would be attached to the dummy load. Switches would connect the load and dummy load to the common side of the pulse forming network. In operation, one switch would be closed connecting the dummy load across the pulse forming network which would start a partial discharge propagating from the dummy end to the load end of the pulse forming network. After a predetermined time, a switch would connect the load to the pulse forming network which would start a partial discharge propagating from the load end to the dummy load end of the pulse forming network. The two pulses upon traveling the length of the pulse forming network would turn off the switch at that end. The spacing of time between the turn-on of the switch connecting the dummy load to the pulse forming network and the turn-on of the switch connecting the load to the pulse forming network determine the pulse time width at the load. One example of switches having the turn-off characteristic when the voltage went below a certain level is the thyratron. In the prior art a variation in pulse width resulted in a variation of the pulse energy delivered to the load. Another prior art configuration of a line type modulator utilized a pulse forming network with a load coupled to one end of the pulse forming network with a semiconductor switch. The switch at an appropriate time coupled a load to the pulse forming network whereupon the pulse forming network discharged across the load providing a fixed pulse width. The other end of the pulse forming network was left open. The pulse width for this configuration was a time for the partial discharge to propagate from the load end to the open end and back or twice the one way propagation time of the pulse forming network.
In the field of high energy laser systems, the need for lightweight high energy pulse modulators with the capability of changing pulse width on a pulse-to-pulse basis has been established. It is therefore desirable to provide a line type modulator which can deliver constant energy from pulse to pulse and at the same time have a stepwise variable pulse width. Furthermore, it is desirable that a line type modulator would utilize all solid state switches which are compact, lightweight, and immune to vibration as compared to thyratrons or other tube switches. It is desirable that the full energy storage capability of the pulse forming network is utilized or delivered to the load regardless of choice of pulse width, especially for laser applications.